Inquisitive, student, teacher and a wanna be story teller. Working on https://learningpaths.io/
I recently read a medium post where the author claimed that using async-await
is better than using promises.
While this might hold good in general cases, I think that generalisation was too broad and doesn’t do justice to either async-await
or promises.
For somebody who is new to Javascript making sense of these and decide which one to use can be a challenge. In this post I will list down things that I have learnt about these and how I decide when to use what. If you find anything interesting/useful in this article you can highlight them using our chrome extension https://bit.ly/highlights-extension
I read somewhere that async-await
is syntactical sugar for using promises
. So before getting to know async-await
or deciding on which approach to you use make sure that you have a better understanding of promises. I captured my learnings in the post Understanding promises in Javascript. If you do not have time to go through the full article checkout the thumb rules that I follow.
resolve
maps to then
and reject
maps to catch
for all practical purposes..catch
and .then
methods for all the promises..finally
Promise
object whether they are static methods or prototype methods is again a Promise
Promise.all
the order of the promises are maintained in values variable irrespective of which promise was first resolved.Once you have wrapped your head around promises checkout async await.
It helps you to write code that is much more readable. When not used properly it has its downsides. You can read Understanding async-await in Javascript to checkout my learnings. If you do not have time to read the full article checkout my thumb rules.
Here are a list of thumb rules I use to keep my head sane around using async
and await
async
functions returns a promise.async
functions use an implicit Promise to return its result. Even if you don’t return a promise explicitly async
function makes sure that your code is passed through a promise.await
blocks the code execution within the async
function, of which it(await statement
) is a part.await
statements within a single async
function.async await
make sure to use try catch
for error handling.await
within loops and iterators. You might fall into the trap of writing sequentially executing code when it could have been easily done in parallel.await
is always for a single promise.await
only blocks the code execution within the async
function. It only makes sure that next line is executed when the promise
resolves. So if an asynchronous activity has already started then await
will not have an effect on it.The answer is that we will use both. Following are the thumb rules I use to decide when to use promises
and when to use async await
async function
returns a promise.
The converse is also true. Every function that returns a promise
can be considered as async function
await
is used for calling an async function
and wait for it to resolve
or reject
.await
blocks the execution of the code within the async
function in which it is located.function2
is dependent on output of function1
then I use await
.async functions
and then run them in parallel.Promise.all(promisesArray)
await
remember that you are writing blocking code. Over the time we tend to neglect this.async functions
with many await asyncFunction()
in it, it is better to create smaller async functions.
This way we will be aware of not writing too much of blocking code.async functions
is that you force yourself to think what are the async functions that can be run in parallel.async
function. By doing this you are making sure that somebody else can use your function asynchronously.Hope this helps you decide easily when to use promises and when to use promises
and when to useasync-await
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This article is a part of my learning series, which I call Understanding X. You can read more above it in this link.
Understanding Javascript
Understanding Linux
Understanding React
Understanding Learning
Understanding Cryptocurrencies